1. Field of the Invention
The present invention relates to a process for reusing overspray resulting from the spraying of water-dilutable two-component polyurethane coating compositions. The term "overspray" is defined as the components which do not reach the substrate to be coated when the coating composition is sprayed and, therefore, are lost if no special precautions are taken.
2. Description of the Prior Art
Solvent-containing two-component polyurethane lacquers have acquired great practical importance due to their outstanding properties. It has recently become known to combine certain polyhydroxyl acrylates (EP-B-0,358,979) or certain polyhydroxyl polyesters (EP-A-0,496,205) with polyisocyanates which contain free isocyanate groups to produce aqueous two-component systems. These coating compositions have a pot life of several hours and harden to high quality cross-linked films which are comparable in their properties to the lacquer films hitherto obtained from solvent-containing two-component polyurethane coating compositions.
In practice, the lacquers are generally applied to their substrates by spraying. It is not possible to prevent significant quantities of the coating components to be lost by overspray. In practice, these excess quantities are generally washed out of the exhaust air of the coating plant with water and coagulated with the aid of coagulating agents such as surface active, inorganic materials, e.g., clay. Previously, there has been no use for this "lacquer sludge."
Processes by which water dilutable one-component systems can be spray lacquered without producing waste have recently been developed. JOT 3/1992 pages 34-39 proposes discs or bands arranged behind the material to be sprayed. The overspray is then continuously stripped off, collected in containers, readjusted to spray viscosity with water and then reused. A possibility of recovering the overspray of water dilutable, air drying lacquers in a coating plant, including washing with water is described in EP-A-0,508,107. The lacquer, which has been diluted with plant water, undergoes no chemical change and is reconcentrated to the original solids content by ultra-filtration.
According to DE-OS 4,138,088, ultrafiltration is also suitable for reconcentrating the overspray of water dilutable two-component polyurethane systems that have a long pot life if polymerization retarders are added. Since the cross-linking reaction of water dilutable two-component polyurethane systems continues to progress at room temperature, the system which has been worked up is not stable in storage but undergoes continuous change, which would be expected to cause considerable fluctuations in quality.
It is an object of the present invention to develop a process which enables the overspray of aqueous two-component polyurethane systems to be worked up into new storage stable coating compositions before it has become completely unusable due to the gradually progressing cross-linking reaction.
This object may be achieved in accordance with the present invention. It has been found that the overspray of coating compositions, which essentially contain a mixture of
a) organic polyisocyanates, PA1 b) an aqueous solution or dispersion of organic compounds containing isocyanate reactive groups, preferably organic polyhydroxyl compounds, and PA1 c) optionally pigments and/or other lacquer additives, can be used again if the polyisocyanates of the overspray are reacted with compounds which are more highly reactive with isocyanate groups than either the isocyanate reactive compounds or water. PA1 (i) collecting the overspray and optionally diluting it with water, PA1 (ii) simultaneously with or after step (i), reacting the overspray, optionally in the presence of emulsifiers, with compounds which are more reactive with isocyanate groups than both water and the compounds containing isocyanate reactive groups and PA1 (iii) reusing the resulting aqueous solution or dispersion of the chemically modified overspray in a one-or two-component, chemically and/or physically drying coating composition, optionally after adjusting the spray viscosity of the coating compositions to a suitable spray viscosity by either removing or adding water and optionally after the addition of additives. PA1 1. alkyl sulphates, especially those having a chain length of 8 to 18 carbon atoms, alkyl and alkylarylether sulphates having 8 to 18 carbon atoms in the hydrophobic radical and 1 to 40 ethylene oxide or propylene oxide units, PA1 2. sulphonates, especially alkyl sulphonates having 8 to 18 carbon atoms, alkylaryl sulphonates having 8 to 18 carbon atoms, taurides, esters and semi-esters of sulphosuccinic acid with monohydric alcohols or alkylphenols having 4 to 15 carbon atoms in the alkyl radical; these alcohols or alkylphenols may also be ethoxylated with 1 to 40 ethylene oxide (EO) units, PA1 3. phosphoric acid partial esters and their alkali metal and ammonium salts, especially alkyl and alkylaryl phosphonates having 8 to 20 carbon atoms in the organic radical, alkyl ether and alkyl arylether phosphates having 8 to 20 carbon atoms in the alkyl or alkylaryl radical and 1 to 40 EO units, PA1 4. alkyl polyglycol ethers preferably having 8 to 40 EO units and alkyl radicals having 8 to 20 carbon atoms, PA1 5. alkylaryl polyglycol ethers preferably having 8 to 40 EO units and 8 to 20 carbon atoms in the alkyl and aryl radicals, PA1 6. ethylene oxide/propylene oxide (EO/PO) block copolymers preferably have 8 to 40 EO units and/or PA1 7. ammonium amphiphiles having a chain length of 4 to 15 carbon atoms in the alkyl radicals and mixtures of these and/or other emulsifiers or surface active substances.